FIELD OF THE INVENTION
The invention relates to methods for monitoring the status of a catalytic converter through which a catalyzable fluid can flow, such as a catalytic converter in the exhaust system of an internal combustion engine, and catalysts for corresponding use that are especially suitable for monitoring according to the method of the invention.
In order to meet the increasingly stringent environmental protection regulations in many countries, internal combustion engines, especially motor vehicle engines, are increasingly being equipped with exhaust systems that are provided with catalytic converters for converting pollutants in their exhaust gases into harmless substances. The term catalytic converter is typically understood to mean a honeycomb-like body of metal or ceramic that has a number of channels through which exhaust gases, or other fluids that have catalyzable components, can flow. Each channel has a wall that is covered with a coating of catalytically active material, which is the catalyst itself. Metal catalytic converters are generally formed of structured sheet metal plates that are stacked, spirally wound, or otherwise inter-twined. Such forms are described in Published European Application No. 0 223 058 B2, corresponding to U.S. Pat. No. 4,824,011; Published European Application No. 0 245 737 B2, corresponding to U.S. Pat. Nos. 4,832,998 and 4,923,109; or Published European Application No. 0 245 738 B2, corresponding to U.S. Pat. Nos. 4,803,189 and 4,946,822, for instance.
In order to assure the function of a catalytic converter in the exhaust system of an internal combustion engine, it is known to provide the catalytic converter with measuring sensors for monitoring the temperature, or the like, so that conclusions can be drawn from the measured values obtained as to its function during engine operation. Proposals along such lines can be found in German Published, Non-Prosecuted Application 26 43 739 A1, German Published, Non-Prosecuted Application 37 10 268 A1, and Published European Application No. 0 236 659 A1. The first two of those references propose equipping a catalytic converter with at least two temperature sensors, which are disposed at different locations in the catalytic converter, one after the other in terms of the direction of the flowing exhaust gas. The measuring sensors may both be disposed outside the catalytic converter, with a first sensor upstream of an inlet end of the catalytic converter and a second sensor downstream of an outlet end of the catalytic converter. German Published, Non-Prosecuted Application 26 43 739 A1 also provides for at least one measuring sensor in the interior of the catalytic converter. Those two references also propose to form a signal from the sensor measurement signals that corresponds to the difference in the temperatures ascertained by the measurement sensors, and to use that signal to evaluate the function of the catalytic converter. In Published European Application No. 0 236 659 A1, an evaluation system for the signals of the measurement sensors is presented. The evaluation system issues a warning signal if the temperature difference exceeds a certain first limit value, and a signal warning of permanent damage if the temperature difference exceeds a second limit value, which is markedly above the first limit value. German Published, Non-Prosecuted Application 26 43 739 A1 also proposes disposing two temperature measuring sensors directly next to one another in the vicinity of the outlet end of the catalytic converter. A first sensor has a catalytically active surface, but a second sensor has a catalytically inactive surface. With such a measurement configuration, it is possible to ascertain if the exhaust gas flowing past the measuring sensors still contains any convertible pollutants. If so, that would cause the two measuring sensors to signal different temperatures.
The prior art offers few suggestions for achieving safe and reliable functional monitoring of a catalytic converter with a view toward aging and poisoning processes. In particular, with the known systems, it is not possible to make a statement as to the status (in terms of aging and/or poisoning) of the catalytic converter that is still functioning adequately overall. Recourse must always be made to very indirect measurement values, such as the temperature of the fluid to be catalyzed before reaching the catalytic converter or after passing through it. Furthermore, the only measurement values that are evaluated are those that were obtained only at certain points on the catalyst and therefore, if for no other reason than because of the generally extremely nonhomogeneous impingement of the fluid to be catalyzed upon the catalytic converter, they are only very limitedly capable of providing accurate information. Additionally, the measurement values typically obtained are highly dependent on the particular strain on the catalytic converter. In order to obtain more reliable statements regarding the status of the catalytic converter, they must necessarily be analyzed in the light of other data. Such data include information that indicate the status of the fluid passing through the catalytic converter, such as the temperature, flow velocity, and content of catalyzable components, for instance.